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1
The Digestive System-Chapters 62-66; 70; 78
Figure 62-1; Guyton & Hall
2
Digestive Processes Ingestion Propulsion Digestion: Mechanical and
Chemical digestion Absorption- nutrients and
water Defecation
3
Layers Alimentary Canal 1. Serosa 2. Longitudinal muscle
(muscularis externa) 3. Myenteric
(Auerbach’s) nerve plexus
4. Circular muscle 5. Submucosa 6. Submucosal
(Meissner’s) nerve plexus
7. Muscularis mucosae 8. Mucosa 9. Epithelial lining
4
Autonomic nerve fibers Both divisions found in myenteric
and submucosal nerve plexi—What do they do? Sensory neurons that monitor
tension, and efferent visceral motor fibers. OWN SYSTEM!
Myenteric-GI motility control- Stimulatory influences -
• tonic contraction (tone) • contraction frequency /
intensity ( propulsion)- Inhibitory influences
• Decreased Sphincter tone (relax) - pyloric sphincter, ileocecal sphincter, LES
Submucosal- Local control- Secretion- Absorption- Contraction of muscularis
mucosa
5
Control of the digestive system Movement of
materials along the digestive tract is controlled by: Neural mechanisms
Parasympathetic (Ach) and local reflexes
Hormonal mechanisms Enhance or inhibit
smooth muscle contraction
Local mechanisms Coordinate response to
changes in pH or chemical stimuli and stretching
6
Digestive Enzymes
Salivary glands-amylase
lingual lipase
Stomachpepsin
Intestinal Mucosa enterokinase sucrase maltase lactase amino-
oligopeptidase dipeptidase
Pancreas amylase trypsin chymotrypsincarboxypeptidaselipasecholesterolesterase
7
The mouth opens into the oral or buccal cavity Its functions include:
Analysis of material before swallowing
Mechanical processing by the teeth, tongue, and palatal surfaces
Lubrication Limited digestion
Lingual lipase (negligible fat digestion)
Salivary amylase (limited carbohydrate digestion)
Antibodies and proteolytic enzymes
Primary SecretionAlpha-amylase
8
Digestion and absorption in the stomach
Short-term storage reservoir
Secretion of intrinsic factor
Pepsinogen gastrin Chemical and enzymatic
digestion is initiated, particularly of proteins
Liquefaction of food Slowly released into the
small intestine for further processing
9
Gastric glands Two types glands -
- Gastric - HCl
(oxyntic) pepsinogen intrinsic
factor mucus
- Pyloric - gastrin mucus
10
Gastric glands- 3 types of cells Mucous Neck cell
(goblet)- release mucus to protect mucosa from acid and pepsin
Parietal cells- HCl and intrinsic factor (B12 absorption by small intestine).
Chief- numerous and release pepsinogen
80%
11
K+K+K+
K+
Na+Na+
H+
Na+ Na+
Cl-Cl-
Cl-
Cl-
H2O
H2O
osmosis
H2O HO- + H+
HCO
3
HCO
3
CO2CO2
C.A. Final ResultsHCl - 155 mEq/LKCl - 15 mEq/LNaCl - 3 mEq.L
pH = 0.8
P
P
BLOOD LUMEN
P
P
Acid production and secretion
12
2 cell types of Pyloric gland G-cells - release gastrin
Enteroendocrine cells -stimulates parietal cells to secrete acid and increases pyloric contraction; relaxes pyloric sphincter
Mucus neck cells - mucous
20%
13
Gastric and Duodenal ulcers
Peptic ulcers occur when damaging effects of acid and pepsin overcome ability of mucosa to protect itself Gastric ulcers - main problem is decreased ability of
mucosa to protect itself
Duodenal ulcers - main problem is exposure to increased amounts of acid and pepsin
Strengthensmucus, HCO3
- secretion, gastrin, PGs, epidermal growth
factor
WeakensH. pylori, aspirin, ethanol,
NSAIDs, bile salts
14
What is the Gastric Mucosal Barrier?
alkaline mucus resists the acid and enzymes Tight junctions-gastric juice can’t seep into
lamina propria Epithelial cell replacement- 3-6 day life span. Physiological - diffused H+ ions are transported back to
lumen
H+ back-leaks into mucosa in exchange for Na+. This is a forerunner to gastric ulcer - Decreased cell pH leads to cell death
Damaged mast cells (ECL cells) leak histamine
Viscous cycle - Histamine .. vascular damage .. local ischemia .. greater leakage of H+.. more cell death ...
Damaged Gastric Mucosal Barrier
15
Helicobacter pylori H. pylori found in 95% patients with DU
and 100% patients with GU (when alcohol, aspirin, NSAIDS are eliminated)
Gram negative bacterium
High urease activity - high NH4+ activity
- can withstand acid environment- NH4
+ damages epithelial cells (GU)- Increases acid secretion (DU)
16
Treatment of Peptic Ulcers Antacids H2 receptor blockers - Rantidine (Zantac)
- Cimetidine (Tagamet)
Proton pump inhibitors - Omeparazole (Prilosec)
Antibiotics
Surgical (rare) - vagotomy- antrectomy
17
Stimulation of acid secretion Gastric secretion is
stimulated by local (distention), neural, and endocrine mechanisms Acetylcholine - HCl
secretion - mucus, pepsinogen, and
gastrin Histamine - HCl
secretion Gastrin - HCl
secretion (1500x more powerful compared to histamine)
Seeing, smelling and anticipating food is perceived in brain. Brain tells stomach to prepare for receipt of mealAccounts for 30% of acid response to meal
60%
10%
18
Small intestine Important digestive
and absorptive functions Secretions and buffers
provided by pancreas, liver, gall bladder
Three subdivisions: Duodenum Jejunum Ileum
Ileocecal sphincter Transition between small
and large intestine
19
Histology of the small intestine Plicae
Transverse folds of the intestinal lining Villi
Fingerlike projections of the mucosa Lacteals
Terminal lymphatic in villus Microvilli
Brush border: increases surface area 20-fold
20
Intestinal glands secretin to stimulate
pancreas to release bicarbonate mucus
cholecystokinin to stimulate pancreas and gallbladder
Gastric Inhibitory peptide (GIP)- inhibits gastrin secretion and decreases stomach emptying
Duodenal glands- bicarbonate mucus.
21
The Activities of Major Digestive Tract Hormones
Figure 24.22
22
Small Intestine- digestive enzymes Maltase- splits maltose into 2
glucose units Lactase- splits lactose into glucose
and galactose Sucrase- splits sucrose into glucose
and fructose Peptidase- breaks down small
peptides into amino acids Intestinal lipase- breaks down
triglycerides into free fatty acids and monoglycerides
Enterokinase- Activates trypsinogen to trypsin (trypsin then activates chymotrypsinogen and procarboxypeptidase)
23
Pancreas As chyme floods into small intestine two
things must happen:
Acid must be neutralized to prevent damage to duodenal mucosa
Macromolecular nutrients - proteins, fats and starch must be broken down much further so their constituents can be absorbed
Pancreas plays vital role in accomplishing both objectives
Digestive enzymes for all food types
Bicarbonate solution to neutralize acid chyme
24
Regulation of Pancreatic Secretion Secretin and CCK are
released when fatty or acidic chyme enters the duodenum
CCK and secretin enter the bloodstream
Upon reaching the pancreas: CCK induces the secretion of
enzyme-rich pancreatic juice Secretin causes secretion of
bicarbonate-rich pancreatic juice
Vagal stimulation also causes release of pancreatic juice
25
The Pancreas
Exocrine function (98%) Acinar cells make,
store, and secrete pancreatic enzymes
Endocrine function – ( cells) release
somatostatin (inhibitory to gastrin and insulin and glucagon)
β-cells –release insulin α-cells-Release glucagon
26
The Pancreas as an Endocrine Gland Insulin
Beta cells Skeletal muscle and
adipose tissue need it to make glucose receptors
Promotes glucose uptake Prevents fat and glycogen
breakdown and inhibits gluconeogenesis
Increases protein synthesis
Promotes fat storage
Picture from:http://www.dkimages.com/discover/Home/Health-and-Beauty/Human-Body/Endocrine-System/Pancreas/Pancreas-1.html
Epi/Norepi inhibit insulin!Help maintain glucose levels during times of stress and increase lipase activity in order to conserve glucose levels
27
The Pancreas as an Endocrine Gland Glucagon
Maintains blood glucose between meals and during periods of fasting.
Nervous tissue (brain) do not need insulin; but are heavily dependent on glucose levels!
Increases blood glucose levels.
Initiates glycogenolysis in liver (within minutes)
Stimulates amino acid transport to liver to stimulate gluconeogenesis
Image from: http://www.dkimages.com/discover/previews/768/74261.JPG
28
Disorders of the Pancreas: Diabetes Mellitus Gestational Diabetes Type I diabetes – develops
suddenly, usually before age 15 Destruction of the beta cells Skeletal tissue and adipose
cells must use alternative fuel and this leads to ketoacidosis
Hyperglycemia results in diabetic coma
29
Disorders of the Pancreas: Diabetes Mellitus Type II diabetes and
metabolic syndrome– adult onset Usually occurs after age
40 Cells have lowered
sensitivity to insulin Controlled by dietary
changes and regular exercise
30
31
Pancreatic Failure
Digestion is abnormal when pancreas fails to secrete normal amounts of enzymes. Pancreatitis Removal of pancreatic head - malignancy
Without pancreatic enzymes - 60% fat not absorbed (steatorrhea) 30-40% protein and carbohydrates not absorbed
32
Pancreatitis Pancreatitis means inflammation of pancreas.
Autodigestion theory can explain condition.
Chronic pancreatitis - (multiple shared causes) alcohol - most common cause in adults cystic fibrosis - most common cause in childre
CF patients lack chloride transporter at apical membrane. Watery ductal secretion decreases which concentrates acinar secretions in
ducts. Precipitation of proteinaceous secretions block ducts and can destroy
gland by autodigestion.
Acute pancreatitis - (multiple shared causes) Gallstones - most common cause
33
Absorption of digested polymers is linked to Salt Absorption in Small Intestine
• Sodium is absorbed across apical cell membrane by 4 mechanisms - 1. Diffusion - through water-filled channels 2. Co-transport - with AA and glucose 3. Co-transport - with chloride 4. Counter-transport - in exchange for H+
• Chloride follows electrical gradient created by absorption of sodium
34
Sodium Absorption in Small Intestine
Na+Na+
Na+
S
S
Na+
Na+Cl-Na+ Cl-
Na+
H+
Na+
H+
PNa+ Na+
K+ K+
Cl-Cl-
1
2
3
4
Aldosterone increasesNa+ reabsorption and K+ secretion in S.I. and colon.
35
Chemical Digestion: Carbohydrates Begins in the mouth (minimal) and
mostly occurs in small intestine when pancreatic enzymes are released
Absorption of monosaccharides occurs across the intestinal epithelia Absorption: via cotransport with Na+, and facilitated diffusion Enter the capillary bed in the villi Transported to the liver via the hepatic
portal vein Enzymes used: salivary amylase,
pancreatic amylase, and brush border enzymes (maltase, lactase, and sucrase)
lumen
36
Chemical Digestion: Proteins Absorption: similar to
carbohydrates (sodium co-transport)
Enzymes used: pepsin in the stomach
Enzymes acting in the small intestine Pancreatic enzymes –
trypsin, chymotrypsin, and carboxypolypeptidase (these must be activated!)
Brush border enzymes – peptidases
37
Lipid digestion and absorption Lipid digestion utilizes
lingual and pancreatic lipases, cholesterol esterase (cleaves ester bond to release cholesterol) and phospholipases release fatty acids and monoglycerides. Bile salts improve chemical
digestion by emulsifying lipid drops
Lipid-bile salt complexes called micelles are formed
38
Fatty Acid Absorption Fatty acids and
monoglycerides enter intestinal cells via diffusion; bile salts can be reused to ferry more monoglycerides
They are combined with proteins within the cells
Resulting chylomicrons are extruded
They enter lacteals and are transported to the circulation via lymph
39
Sprue• Diseases that result in decreased
absorption even when food is well digested are often classified as “sprue” - - Nontropical sprue - also called celiac disease
- allergic to gluten (wheat, rye)- destroys microvilli and sometimes
villi- Tropical sprue - bacterium (?)
- treated with antibacterial agents
• Steatorrhea - if stool fat is in the form of FFA - digestion has occurred
40
Stomach(2)
Fluid Entering and Exiting the Gut
Bile (1)
0
2
4
6
8
10
Vol
ume
(L/d
ay)
Diet (2)
Pancreas (1)
Saliva (1)
S.I. (2)
Duodenum and
Jejunum (4)
Colon (1.4)
Ileum(3.5)
Volumeentering
Volume absorbed
Volume Excreted
100-200 ml
•95% of water is absorbed in the small intestines by osmosis•Water moves in both directions across intestinal mucosa•Net osmosis occurs whenever a concentration gradient is established by active transport of solutes into the mucosal cells
41
The Liver Digestive function – bile
production; emulsifies fats Bilirubin- decomposed
hemoglobin Urobilinogen- by-
product of bilirubin metabolism
bile salts- keep cholesterol dissolved in bile
Performs many metabolic functions- stores vitamins, processes fats, detoxifies,
makes blood proteins
42
Physiology of the large intestine
Reabsorption in the large intestine includes: Water and electrolets Bacteria make: Vitamins –
K, biotin, and B5
Organic wastes – urobilinogens and sterobilinogens
Bile salts Toxins
Mass movements of material through colon and rectum Defecation reflex triggered
by distention of rectal walls
Figure 8-18 Agents that stimulate and inhibit H+ secretion by gastric parietal cells. ACh, Acetylcholine; cAMP, cyclic adenosine monophosphate; CCK, cholecystokinin; ECL, enterochromaffin-like; IP3, inositol 1,4,5-triphosphate; M, muscarinic.
Downloaded from: StudentConsult (on 23 April 2010 06:51 PM)© 2005 Elsevier
Figure 8-19 Regulation of HCl secretion during cephalic and gastric phases. ACh, Acetylcholine; GRP, gastrin-releasing peptide (bombesin).
Downloaded from: StudentConsult (on 23 April 2010 06:51 PM)© 2005 Elsevier
Figure 8-20 Balance of protective and damaging factors on gastroduodenal mucosa. H. pylori, Helicobacter pylori; NSAIDs, nonsteroidal anti-inflammatory drugs.
Downloaded from: StudentConsult (on 23 April 2010 06:51 PM)© 2005 Elsevier
Figure 8-15 Secretory products of various gastric cells.
Downloaded from: StudentConsult (on 23 April 2010 06:51 PM)© 2005 Elsevier